Method for electrolytic stripping and determination of plating metal



Dec. 7, 1948. G. H. BENDIX 72,455,726

METHOD FOR ELECTROLYTIC STRIPPING AND 4 DETERMINATION OF PLATING METAL Filed July 13, 1942 grave/whoa Q Wags Patented Dec. 7, 1948 UNITED s r-Ares;

* PATENT OFFICE METILQD non g Lnc'monY'rrc s'mn PING AND DETERMINATION OF-PLAT- ING METAL Gordon H.'Bendix, Chicago, 111., assignortol Con: tinental'CangCompany, Ina, NewYork, N, Y., a

it corporation of New York Applicationlul y 13, 1942, Serial N'o.'45 *,750 i 4 Claims.

inventiomrelates to the strippingof mu1ti-' valent plating meta-1f from material; and is particularly concerned-withsuch stripping as means of determining; the amount of plating metal:

which is present inthe plate d material.

;It has heretoforebeen propbsedto employ electrolytic stripping for-removing the plating metal through an oxidizing operation, with the utilization offan electrolyte of oxidizing character, 01-

lowedby a determination of the decrease ofjoxidiz ing power of thebody of the electrolyte: but such prior proposals have, been coupled with the difii'culty that these determinations were net siqnp aniappar tus. and me ho by .wh i he determination can be lef f ected uponthe entirety orj tandardized pecimen of. p at d sh ei tl i a p I simpl iand c nvenient. mann ra t t a A further featureof the invent ion;is the pro? visioni of t i an app ratus and me ho whi ated-met z vina coatin s on b th side gmer esub e t d:taa ne e pon ratiqn or dete minin the amount of plating metal present, without equ re n e mnar on t ta ei st bla ks,

ndwi hw req rd na as anda t meoi opera: tion. j 7

With these and other features as objects in view, an illustratiyeform-of practicing the inentipn; i se r -i e;n ow ead s riptio in association with the drawing-s,2 in w'hich;

Figure 1 is an upright sectionalview through an electrolytic stripping apparatus, comprising a container and; stand and-the associatedtstructures for performing theprocessy beinghtaken substantially on,1ine l l of Fig Z.

Figure 2 is a sectional View of" the container wit i sias e tedl r ak n; ubs ntia y n line; 2 2, of -l iighl. i

In these drawings and thereof is set out a preferred form of employing the inventionin determining the weight on E detailedz qeseri t e i thicknes of t the tin coating in a normal tin plate. For this purpose, a specimen i-s prepared which hasadefinite area,,and ismade the anode between two carbon cathodes which, are shutoff 5 from the anode bydiaphragm means. In practice,, a round specimen 2:118 inches in diameter (4 lsquare inches area, oneside) is a desirable form. Surrounding th e specimen isran anolyte comprising a: solution ot iodine and potassium,

s; iodide in dilutehydrochloric acid. The catholyte inlnthis preferred i-formiof practice is. dilute hydrochloricqacid. Upon. passage of a direct current at a -current densityof about three amperes (i. e. three-eighths ampere per square inch, iorthetwo a sideshfon about-two minutes, the tin is stripped froma specimen-0t the tin plate; as employed for forming food containers this tin passing initially intosthe solution in the stannous state. During this stripping of the tin, a chemically equivalent amount; of hydrogen ionsis discharged at the cathodes and passes oifinto the atmosphere without coming into contact with the iodine solution. secondarily, the iodine in the anolyte react-smith thetstannous tin to oxidize it to the stam1ic state; and: is" itself reduced to iodide.

'liherquantityr-of iodine thus converted toiodide is-directly proportional to the quantity of tin dissolved; and hence a determinationz of the decrease of elemental iodine content from the beginning rof; the electrolysis to the end of the electrolysis perm-its computation of the amount oftin whichrhasabeen trippedfrom the tin plate. Itw-hasbeendound in, practice that no significant interfering reactions v occur at the anode or-in the anolyte, and that the segregation of the eatholyte; so that cathodehydrogen is prevented'i from contact with the anolyte or with theanode, permits direct determinations rapidly and accurately.

n In the apparatus shown on the drawings, a stand H) has a narrow elevated ledge H, a projecting finger n, anda shelf I3. A container" B; such as'the- 400 millimeter glass beakerof standardvtype; may be introduced to the stand bycengagingitslip with the finger I 2 and then rocking the beaker into position so that it can rest onw ledge: H; i no The shelf I3 is apertured to receive and supporttheltwoeporous cups I'd-having the enlarged 'ff1anges. lfiythe; cups 1 li beingtwithin the container B when the latter is mounted on the stand. A metalsupport I1 is secured to the back wall of the stand l and has a vertical aperture through which may be slid the iron pole piece I8, which passes through a small aperture in the shelf l3 and thus into the container B at a point between the two porous cups IS. The porous cups [5 receive the stoppers I9, which may be of glass and in which are mounted carbon rods 20 constituting cathodes which are thus positioned within the cathode compartments formed by the porous cups IS.

A conductor 2| is connected to the two cath odes 20 and extends to the negative terminal of a source 22 of direct current, and a conductor 23 extends from the source 22 and is connected, in this preferred form, to the support IT. The current flowing may be read by an ammeter A included in one of these conductors. Abranch 2 3 from the conductor 23 includes a rheostat 25 and supplies current to the winding 26 which is positioned around the pole piece l'8, with a current return by conductor 21 to conductor 2| The determinations are effected by preparing a specimen S which is illustrated as being a disc cut from the material to be tested. The pole piece I8 may be adjusted vertically'and secured by the screw Ila so that its lower end is at or slightly below the standard liquid level in the container B. The current from the batter 22 flows by the conductors and winding 26 so that the pole piece 18 is magnetized, and is thus capable of su portin the specimen S by its edge in totally submerged conduction in the standard volume of solution in the container B.

During the course of testing, the lower end of the pole piece l8 may be eaten away, but it is then a simple matter to relieve the screw Ila, and adjust the pole piece downwardly.

The container is provided with a standard quantity of reagent solution, and then is introduced into the stand, so that the liquid level is above the top of the specimen S. The flow of current through the conductors 2|, 23 causes electrolysis within the cell so that the aforementioned reactions occur therein.

Preliminary to the determinations, certain re-- agents are prepared as stock solutions. The concentrations relative to iodine should be between 0.2 and 0.01 normal. 'At the higher end of this range, accurate titration becomes difilcult; and at the lower end, the starch end point is sometimes indistinct. the reagents are:

A N/ 10 normal potassium iodate solution is prepared by dissolving 3566 grams of potassium iodate, 22 grams of potassium iodide, and 1 gram of sodium hydroxide in water made up to a volume of 1 liter. The alkali serves to prevent hydrolysis of the iodate. This solution must be made up accurately with respect to the weight of potassium iodate per volume, or as an alternative it may be standardized upon titration against a suitable standard such as arsenic trioxide.

A N /l0 normal solution of sodium thiosulfate is prepared by dissolving 25 grams of crystalline sodium thiosulfate (Na2S2O3.5H2O) with 0.1 gram of sodium carbonate in water to make up 1 liter. This solution is about N /10, and is then standardized against the N/10 potassium iodate solution above.

A dilute hydrochloric acid solution is prepared by diluting concentrated (1.19 sp. gr.) hydrochloric acid with four to ten times its volume of water: the present preference is a ratio of 1:7.

Examples of good practice for A starch indicator of 0.2 percent strength is prepared by dissolving 2 grams of soluble starch in 1 liter of boiling water, and cooling. A trace of a preservative such as chloroform or mercuric iodide may be added.

These solutions will keep for a considerable period of time, and need not be freshly prepared immediately prior to use.

In preparing for tests, dilute hydrochloric acid is introduced into the porous cups, and a body of anolyte is placed in the container I4. This anolyte is prepared by introducing approximately 200 milliliters of dilute hydrochloric acid into a 400 milliliter beaker and adding an accurately measured quantity of 20 milliliters of the N/lO normal potassium iodate solution. A reaction then occurs:

l8 and is suspended thereby. It will be noted that it thus acts as an armature, but only makes contact at limited portions and at its periphery, so that the tin plating is fully'exposed to the anolyte. v

The beaker with its content of anolyte is then raised into position and brought to rest on the ledge ll.

Current is then allowed to flow at a rate of about 3 amperes for a specimen having a standard area of 4 square inches, for a time of about 2 minutes with tin plate as employed for food containers. The amperage and time may be varied within fairly Wide limits but care, on the one hand, should be taken that sufilcient current passes to dissolve all of the tin from the sample. With this preferred practice, about 1 minute is required for stripping the tin, and an extra minute is allowed for safety. On the other hand,

' the cell should not be operated longer than about (2) Sn-2 electrons- Sn++ At the anode, after the tin has been dissolved, a reaction occurs:

(3) Fe-2 electrons- Fe++ At the cathode, hydrogen ions are discharged and hydrogen gas is formed:

(4) 2H++2 electrons Hz The stannous tin ions formed at the anode by Reaction 2 are oxidized by the iodine according to the following reaction:

V m aicbjaddit a;

ghiflcant recation y betweerr thi by iituitatioii ert v andtlreiferrousioiisfornied-by Reacti'o of cathode hydr'oge" up"arrthe electrolyte unle lie cellis peratedfor"an'excessively lo whicli' th exactnesseof' determination of the detirii'e""K6'-*miiiiites" rmore"for' th"above*'ton creased oxidizingpo er of the electrplyte would tions), or at anl exm'ssiuelyihigltieurrent density. 5 otherwise be obscured? The electrolytic operation removes the, tin I laimth-lrw 1: 1. M. 1- 1 plating arid also any tin .wli'i hfis presenttinfthe" 1. The method of determiningvithewamounttotiin for'iiihfa tiri i'rbrialldy. tin present inwtintplate.having a. ferrous base,

The beaker aridl itsteoiit'eritstare' ndiv removed, which comprisesinagnetically'suspendinga speciandntitrationiis.accomplishedtwith..Nfllosodium man o tin plate by itsr-redg'e in an anolytellit thiosulfate solution whichi is ,add'edgslowlyirroni a ooritj hinge-a 'standard quantity f'of elemental buretteuntil thetipdinefsolution has onlyra lig'h't' iodiii in ith pi esencetothydrochloric acid-, x'pas'sh straw coldri anew'millilitersfoftstarchindicatort ing ectl'ic. urlrentwithsaidispecimen as anode P solution are stirred and the addition of sddium' and emplbyin a separate catholyte' 'ior preve'r'i .3 thiosulfate is continued until the solution changes ing contabt irpit the .cathodeimhydro'gen with n the color from blue to water white upon the addition electrolyte, and determining the amount of iodine of a single drop. consumed during the stripping as the direct indi- The amount of tin can then be computed from cation of the amount of tin originally present on the volume of sodium thiosulfate required for the the tin plate. reduction of the iodine solution, as the decrease 2. The method of determining the amount of of oxidizing power of the iodine solution is ditin present in tin plate having a ferrous base. rectly related to the quantity of tin which has which comprises magnetically suspending a specibeen oxidized from stannous to stannic condition. men of the tin plate by its edge in a standard As an example of such computation, with a quantity of anolyte prepared by mixing a solution P e e of tin plate having an area of 4 square 2 substantially deci-normal in potassium iodate inches (one side), the potassium iodide solution and containing substantially 22 grams of potas- (iontains 31142 r s 3 p r er d has sium iodide per liter with a solution containing an Oxidation normality (molecular We ht o concentrated hydrochloric acid to the amount of KIO3=214.0) of (3.442X6)+214.0==0.0965 N (or 180 milliliters of acid per liter of anolyte, said about mixed solutions containing hydrochloric acid p ontrol analysis of a standard amount, beyond the quantity for reaction with the iodate, c as p y above in t b r it s passing electric current with said specimen as found that the 20 milliliters of K103 solution reanode and at a current density of substantially quires 21.1 milliliters of standard sodium thioampere per square inch of total specimen area su e o ut o e or el ctrolysis; so that 1 for a time of substantially 2 to 6 minutes and until milliliter of sodium thiosulfate solution is equivan th t a d essentially no base metal has enlent to 0.948 milliliter of K103 solution. i tered the anolyte, with the employment of a With the Particular sp i n. f assumpti cathode within the separated catholyte whereby i l it s of e z a so t w q i to prevent contact of the cathode hydrogen with for titreting t iodine after electrolysis of the the anolyte. and determining the amount of iodine sp cimen. Therefore, 21.l0-2.4'7=18.63 milliconsumed during the stripping as the direct indi liter of Nazssos solution represents the a e cation of the amount of tin originally present on by reason of employment of iodine for oxidizing the tin plate.

the stannous tin. This quantity is equivalent to 3, h ethod of determining the amount of 18.63 0.948. l7.66 milliliters of K103 solution. tin resent in tin plate having a ferrous base, Since this solution was 0.0965 N, and was effective which comprises magnetically suspending a specito increase the valency of the tin by 2 electrons men of th tin plate by its edge between two per molecule it can then be computed? catholyte chambers and in an anolyte mechan- 5U ically separated from mixture with the catho- LL 1 lyte, said chambers being individually located 17.66X0'0965X 2 1012 mg' of mm opposite the faces of the tin plate specimen, said anolyte comprising a standard quantity of iodine where 118.7 is the atomic weight of tin. and containing potassium halide and hydros p imen of tin 4 q re inche with its chloric acid, the catholyte chambers being effeccoating of 101.2 milligrams of tin corresponds to tive to prevent contact of the cathode hydrogen ds of tin per b e which s. t e with the anolyte,passing electric current with said standard of quantity in computing the weight of specimen as anode, and determining the amount t coating o a yi of iodine consumed during the stripping as the This computation need not be made e h ti e, no direct indication of the amount of tin originally but a table can be constructed in which the present on the tin plate.

weight of tin per base box is indicated as a direct 4. The method of determining the amount of function of the volume of sodium thiosulfate retin present in tin plate having a fe o s ba e, quired for reducing the remaining iodine after which comprises magnetically suspending a specithe eleCtrOlysiS. men of the tin plate by its edge. electrolytically The invention therefor provides a simple and stripping the tin from the tin plate specimen as convenient pparatus and: method for the quic anode in a hydrochloric acid solution containing determination of the weight of tin coatings. a known quantity of iodine and potassium iodide While the invention has been described in deand in the absence of potassium iodate, and with tail in an application to evaluating the weight 7 a separated catholyte for preventing contact of and thickness of tin coatings upon tin plate, it the cathode hydrogen with said solution; removwill be understood that it can be employed more ing the solution after all the plating metal has generally to the evaluation of multivalent plating been stripped and prior to essential removal of the metals, such as tin, upon ferrous and other bases, base metal of the tin plate; and determining the with employment of proper stripping electrolysis, amount of iodine consumed during the strippin 7 as; the direct indication of the amount, of, tin

Number originally present on the tin plate. T 2,044,888

. GORDON H. BENDIX. 2,047,41 r

2,206,028 REFERENCES CITED 5 The following references are of record in the file of this patent: gg g UNITED STATES PATENTS Number Name Date 10 442,333 Roberts D80. 9, 1890 1,168,280 Buch Jan. 18,1916 1,235,202 Hartsuch July 31, 1917, 1,287,156 Whitehead Dec. 10, 1918 1,724,928 Hailwood Aug. 20, 1929 15 V Name Date Overclich et a1 June 23,1936 Kronsbein July 14, 1936 Buser July 2, 1940. FOREIGN PATENTS Country Date Great Britain May 13, 1938 OTHER REFERENCES Treatise on Inorganic andTheoretical Chemistry, vq]. II (1922), page 315.

Journal of the Electrodepositors' Technical Society, vol. XI (1935-36), pp. 16, 1'1, 20. 

